Waste containment is a major engineering challenge. Both municipal and industrial wastes, if not properly contained or neutralized, can pollute the environment endangering public health via overland flow and via ground water diffusion of the leachable constituents. This also applies to reuse options for waste materials in construction applications such as road base and embankment.
Leachable contaminants can be isolated from the environment using a macro or micro-containment system.
In a macro-containment system, untreated wastes are surrounded by a liner to isolate the hazardous constituents from the environment. Liner systems (this term also includes covers) can either be constructed from synthetic materials, geotextiles, or from natural materials, i.e. low permeability clay or cementing mixtures. Both approaches have limitations. Synthetic liners are practically impervious but they are prone to puncturing when installed, or as a result of differential settling, and their life span in the ground is limited. Natural liners are more durable but they have a finite permeability. Furthermore, their permeability can increase upon contact with leachares. Neither macro-containment system is thus fully reliable for longterm disposal scenarios. The literature on the subject suggest that the life expectancy of synthetic membranes (liners, covers) is in the order of 50 years. This length of service can be extended indefinitely only when remedial measures (repair or replacement) are possible. This is, however, very difficult and expensive once the site is filled with waste material.
Micro-containment systems make use of precipitation/adsorption reactions to chemically immobilize hazardous constituents and often incorporate them in a solidified mass through the addition of various cementing additives. Resulting solid product can often be used in a construction. However, given the complex chemical nature of many waste materials, it is difficult to immobilize all contaminants. The result is that many solidified wastes must still be treated as materials requiring further containment. Furthermore, wastes treated by these methods are not available for future recovery if market or regulatory conditions make that desirable.
From EP-A2-0 390 297 a method is known for sealing off a mass of waste or stabilized waste which comprises:
surrounding the mass of waste, stabilized waste, on its emplacement site with at least one substantially continuous layer of porous material, the layer defining a substantially continuous, waste-encompassing interface. In that specification a cement containing mixture is provided on the waste material. During exposion to the environment the outer surface of the layer will harden. If because of some reason cracks occur in the impermeable hardened outer layer a fresh part of non-hardened material will be exposed to the environment and harden as yet providing in compensation for the cracks occurring.
U.S. Pat. No. 4,981,394 issued Jan. 1, 1991 to Mc Laren et al. and assigned to Terran teaches a composite of material upon or below the earth's surface and above the water table to prevent or effectively inhibit the flow of groundwaters through such layers of initially unconsolidated materials. Chemical components are added to materials such as incinerator ash or other waste materials deposited in a landfill operation to create alkaline conditions which, when contacted by water, induce precipitation of calcite in sufficient quantities to produce a cementing action which retards leachate formation. The patent further describes the preparation of a site as a landfill by choosing a site with alkaline soils or carbonate bedrock and adding appropriate chemical components to cause calcite to precipitate when contacted by water, which may be applied directly or by natural precipitation.
It appears from the patent that previously mixed components are used, or a site with alkaline soils is chosen.
U.S. Pat. No. 4,615,643 issued Oct. 7, 1986 to Gouvenot and assigned to Soletanche, France, proposes to seal off a mass of waste in soil by filling up hollows and cracks in the soil with a grout containing cement, clay, a siliceous product, sodium carbonate and alkali-metal tartrate. The grout would retain heavy metal cations that are leached from the mass of waste.
U.S. Pat. No. 4,726,710 to Rosar et al, Feb. 23, 1988 discloses a process for disposal of fossil fuel ash by adding there to sodium salts, preferably Na.sub.2 SO.sub.x. The co-disposal renders the mixture of salt and ash impermeable.
U.S. Pat. No. 4,456,400 to Heide, et el. (Jun. 26, 1984) describes a process for disposing of waste materials by mixing them with brown coal ash and water to form a flowable pulp, hardenable to an impermeable material upon disposal.
U.S. Pat. No. 4,432,666 to Frey, et al, (Feb. 21, 1984) proposes to form a compact, water-repellent mass of waste by mixing the waste with a binder such as calcium hydroxide, gypsum or cement, followed by solidification, granulation, another mixing with binder and deposition of the pourable mixture in a repository to finish-harden therein.
As indicated hereinabove, one must assume that, realistically, any liner will fail during its lifetime. This applies to both synthetic membranes and to natural materials. Accordingly, there is still a need for a "self-healing" liner, one that will repair itself in situ when damaged.